Quench bath regeneration system for microporous film production

ABSTRACT

Method of forming a microporus film by casting a solution of a film-forming polymer and quenching the film in a non-solvent system quench bath wherein the polymer is preferably a nonalcohol-soluble nylon polymer and the solvent is formic acid. The contaminated non-solvent system is then regenerated by contacting it with a heterogeneous esterification catalyst in the presence of methanol to form methyl formate and then removing the methyl formate by volatilization and recycling a portion of the treated non-solvent system to the quench bath.

United States Patent L/RESERVOIR /35 FILTER 62 our-men BATH 3 LEVEL CONTROL FLUIDIZED ION EXCHANGE RESIN POROUS POLYETHYLENE TUBES REGENERATION BATH Knight et al. Dec. 23, 1975 [54] QUENCH BATH REGENERATION SYSTEM 3,044,115 7/1962 Craver et a1. 264/ 183 FOR MICRO O FILM PRODUCTION 3,173,862 3/1965 Clements et al....... 264/179 X 3,207,828 9/1965 Petersen et a1. 264/184 X Inventors: Rodney g New Mllford; 3,284,554 11/1966 Riley 264/38 x Joseph A. Zang, Cos Cob; John 3,284,554 11/1966 Riley 264/179 Slanski, Trumbull, all of Conn. 3,303,634 2/1964 Berrian 55/431 X 3,324,634 6/1967 Brahler et a1... 55/431 X [73] Asslgnee: AMF Inmrporated, whlte P131118, 3,389,206 6/1968 Jamison 264/203 3,429,957 2/1969 Merten 264/49 3,758,457 9/1973 Broeck et a1. 264/37 X [22] 1973 3,767,737 10/1973 Lundstrom 264/298 x [21] App]. No.: 427,743 3,824,049 7/1974 Schindler.... 264/216 X R24,689 8/1959 Koller 264/ 184 R24,691 8/1959 Steuber 264/184 [52] US. Cl 264/38; 203/38; 260/488 R;

264/41' 264/179 264/DIG. 51 Prima E p And ry xammer- 11p erson Clu Attorney Agent or Firm George W Price; Charles J [58] Field of Search 264/184; 264/216; 260/37, worth 260/38, 184, 488 F, 2.1 R, 448 R; 264/38,

37, 179, 183, 184, 203, DIG. 51, 216;

210/500; 55/431, 467; 203/38 [57] 1 ABSTRACT Method of forming a microporus film by casting a so- '[56] References Cited lution of a film-forming polymer and quenching the UNITED STATES PATENTS film in a non-solvent system quench bath wherein the polymer is preferably a non-alcohol-soluble nylon polgzzgz: ymer and the solvent is formic acid. The contaminated /1934 Crane et a1 264/203 non-solvent system is then regenerated by contacting 2,037,292 4/1936 weiheh 264MB X it with a heterogeneous esterification catalyst in the 2,072,145 3/1937 Wells 264/179 x Presence, of methanol to form methyl formate and 2,360,406 /1944 Dreyfus et a]. 264/184 x h n removing the methyl formate by volatilization 2,367,493 1/1945 Fordyce et al.... 264/203 X and recycling a portion of the treated non-solvent sys- 2,628,249 2/1953 Bruno 260/488 R X (em to the quench bath, 2,708,617 5/1955 Magat et a1 264/179 X 3,023,183 2/1962 Nelson 260/488 R x 14 C a ms, 1 Drawing Figure v Z6 x \r T/ v f v /0 3 COMPRESSED AIR SUPPLY 3 22 1 f z4- S J 32 I8 1 34 J 1 14 MAKE-UP n METHANOL 4/ l l o f H 36 8 3 oueucn earn 2 001.0 BATH I/ QUENCH BATH REGENERATION SYSTEM FOR MICROPOROUS FILM PRODUCTION Cross Reference: The present invention is an improvement of the invention in commonly assigned, copending application Ser. No. 380,403 filed July 18, 1973 now U.S. Pat. No. 3,876,738.

This invention relates to an. improved method of forming film by removing formic acid contaminant from a non-solvent system used to quench the cast film. More particularly, it relates to controlling the amount of formic acid and methyl formate in the quench bath during the production of films. I

As described in the aforesaid copending application, during production of films from solutions of film-forming polymers in a solvent system comprising formic acid, the polymers are extruded ,or cast into a quench bath consisting of a non-solvent system preferably comprising methanol and water. Eventually, the bath becomes contaminated with the polymer-solvent system and the presence of the said solvent system adversely affects the quenching capacity of the bath which necessitates replenishment of the quench bath constituents either by careful controlled addition or by discarding the contaminated bath and replacing with a fresh .quench bath. Alternatively, the quench bath can be freed of contaminants by removal of the undesired solvents, as by fractional distillation or by classical ion-exchange techniques, from portions of the bath and recycling of the thus treated portions to the quench bath. Conventional ion-exchange techniques are expensive and not very efficient, while distillation requires heat, cooling apparatus and substantial apparatus, particularly in contrast with the requirements of the hereindescribed invention.

In the preparing of films by casting or extruding a solution of a film-forming material in a solvent system, the films are usually cast or extruded onto a surface which is generally a moving drum or a moving endless surface. Depending on the film to be formed the cast or extruded film may be exposed for a predetermined time to an atmosphere before the film is brought into contact with a quench bath or the cast or extruded film is immediately immersed in the bath. By controlling certain variables such as the film-forming materials, the solvent system of the casting solution, the concentration of the film-forming material in the solvent system, the quench bath composition, time of quenching, exposure of the film to an atmosphere prior to immersion in the bath, bath temperature, additives in the quench bath and/or the solvent system, the properties of the resulting film can be altered to desired values.

The present invention is particularly useful in filmforming methods involving the casting of a film from a dope solution of a film-forming polymer in a solvent system comprising formic acid and the quenching of said film in a quench bath comprised of a non-solvent system comprising methanol for a time sufficient to form the desired film. The solvent system is comprised of at least one solvent for the polymer and may contain one or more non-solvents for the polymer, the amount of non-solvent being insufficient to cause precipitation of the polymer, and the nonsolvent being miscible with the polymer solution. Preferably the polymer is a nonalcohol-soluble nylon polymer. Such polymers are commercially available under the name Plascon (Allied Chem.) and Zytel (Dupont). It is known that nylons will dissolve in solutions of acids in which it behaves as a polyelectrolyte. Such acids include for example, formic, acetic and similar acids which react with through protonation of nitrogen in the amide group characteristic of nylon. Generally, the non-solvents can be water and/or lower alcohols, such as methanol and ethanol.

The formation of the film can be accomplished by any of the recognized methods familiar to the art. Although any suitable apparatus may be employed, the apparatus shown in the aforesaid copending application is preferred.

In accordance with the present invention there is provided a method of preparing a film by casting a film from a dope solution of a film-forming polymer in a solvent system comprising formic acid, quenching said film in a quench bath comprised of a non-solvent system preferably containing methanol, contacting said quench bath with a heterogeneous esterification catalyst in the presence of methanol to form methyl formate and removing methyl formate from the bath solution. In a preferred form of the invention, predetermined amounts of the non-solvent system of the quench bath are treated in a regeneration system with the heterogeneous catalyst system to form methyl formate which is removed and the treated non-solvent system is recycled to the quench bath. The preferred method is easily adaptable to a continuous process by recycling predetermined amounts of the treated nonsolvent to the quench bath while adding predetermined amounts of contaminated non-solvent to the regeneration system.

Although, at present, the preferred quench baths comprise methanol, methanol is not absolutely essential for forming the film and need not be present in the quench bath, or may be present in only small quantities. In such quench baths, the amounts of methanol can be adjusted to the level required for esterification of formic acid by addition of methanol to the quench bath as required. The amounts of methanol required are determined by the amount of formic acid present in I the quench bath, in turn determinable by chemical analysis.

For maximum efficiency, it is preferred to avoid the presence of higher, esterifying alcohols in the regeneration of the non-solvent system which alcohols would compete with methanol in esterifying formic acid under the conditions employed. The higher alcohols would give rise to less volatile esters in comparison to methyl formate and would necessitate more vigorous conditions and time-consuming removal, for which reason they are preferably avoided. However, the presence of minor amounts of such alcohols-is usually not critical and can be tolerated. For example, ethanol and the propanols can be used as the esterifying alcohol to remove formic acid from the quench bath, but a much higher temperature is necessary to volatilize the resulting esters. While the use of such alcohols is broadly contemplated by the present invention, methanol is preferred for the obvious economy of operation as well as ease and efficiency of removal of methyl formate.

The temperature conditions selected are not critical and usually are those which are compatible with the esterification reaction, the catalyst and the removal of the ester. The esterification and removal of the ester is usually carried out at temperatures from ambient to below the boiling point of methanol, preferably slightly above the boiling point of the ester, e.g. about 35C. When the methanol-containing bath solution is being treated in the regeneration system and recirculated to the quench bath, the selected temperature and flow rate should be controlled to maintain the treated quench bath desirably at optimum levels to obviate any adverse effect of the continuously added formic acid from the polymer dope solution.

Heterogenous esterification catalysts are those esterification catalysts which are immobile with reference to the treated system and can be readily removed from the bath when indicated. Generally, the bath solution should not be readily contaminated by such catalysts to avoid the need of further purification of the non-solvent. Such catalysts are easily recognizable by those skilled in the art. Moreover, the catalysts may be comprised of more than one type, i.e. a mixture of catalysts may be used. The preferred catalysts are either strong acid or strong base ion-exchange resins which are known to catalyze esterification reactions. Of these, the acid ion-exchange resins are preferred since they require shorter reaction time. Such resins are commercially available from the'Diamond Shamrock Chemical Company under the designation Duolite ES-26 as well as from Rohm & Haas under the designation Amberlyst 15.

As with any heterogeneous catalyst, fluidizing of the catalyst is preferred to obtain maximum efficiency and fluidizing with an inert gas is particularly preferred since the gas can also serve to purge methyl formate from the treated bath. In the preferred form of the invention, the flow of the inert gas is sufficient to fluidize the ion-exchange resin and to purge ester from the bath. In addition, preheating the inert gas can also serve to heat the bath solution.

The ester can be removed from the bath solution by stirring and/or passing an inert gas through the bath solution. Although any gas may be used which does not interfere with the reaction and does not contaminate the treated bath solution, e.g. nitrogen, helium, argon, carbon dioxide, and the like, air is preferred because of cost and ready availability.

In utilizing the present process, it is readily possible to regulate the amounts of formic acid in the quench bath at any desired concentration level. For most purposes, it is sufficient to maintain the formic acid concentration at or about 1% by volume of the quench bath, although concentrations up to about 5% by volume or even higher, can be tolerated. The efficiency of removal of formic acid can be increased to regulate this concentration to even lower concentration levels, i.e. less than 1%.

DESCRIPTION OF THE DRAWING In the accompanying single FIGURE of the drawing, there is illustrated a quench bath regeneration system wherein the present invention may be employed during the production of film.

The regeneration system includes a tank 12, preferably of rectangular construction forming a regeneration bath, and having an upper hooded portion 14. A plurality of tubes 16, formed of porous polyethylene, are positioned proximate to the bottom of tank 12 and communicate with a supply of compressed air through a conduit 18. Each of the tubes 16 are covered with one cubic foot of Duolite ES-26 cation exchange resin in the H+ form.

The upper end of hooded portion 14 extends into a duct 20 which communicates through a hood 22 with a quench bath tank 24, and with an exhaust duct 26. A conduit 28 at the bottom of tank 24 connects with an inlet 30 at the upper side of tank 12 so as to form a quench bath level control. The quench bath tank may contain a suitable rotating drum 32 and take-up drum 34 similar to that in structure and operation described in copending application Ser. No. 380,403 filed July 18, 1973 now US. Pat. No. 3,876,738.

Regeneration bath tank 12 has an overflow conduit 36 extending into a reservoir 38, which includes an outlet and return conduit 40 leading into tank 24. The condduit 40 includes a filter 42 and a pump 44 for conveying regenerated quench bath flow liquid back into tank 24 through a control valve 46 and flow meter 48, while a portion of the flow is recycled to reservoir 38 through a branch conduit 50 and valve 52. The conduit 40 also includes a cold bath 45 to lower the temperature of the returning regenerated quench bath and a deaerator 49.

At the inlet side of tank 24 with respect to return conduit 40, the tank includes a labyrinth barrier 54 for the regenerated flow emanating from conduit 40 so as to prevent any turbulence within tank 24 near rotating.

drum 32.

In operation regeneration tank 12 is filled with decontaminated methanol quench bath solution, while air from conduit 18 is forced through tubes 16 so as to create a fluidized ion exchange resin bath, thereby providing an extremely high contact surface between the resin and the solution in the regeneration bath in tank 12. The formic acid in the solution is converted to methyl formate and removed from the bath by the air purge. The methyl formate vapors are exhausted through hooded portion 14, duct 20 and duct 26. The decontaminated methanol quench bath solution then flows through reservoir 38 and conduit 40 back into quench bath tank 24. Contaminated methanol quench bath solution at the bottom of tank 24 is coveyed through conduit 28 and inlet 30 into tank 12 for air purge treatment in a continuous cycle. Because methanol liberates large quantities of finely divided air bubbles when mixed with water or mixtures of water and methanol, the regenerated quench bath solution, which is over saturated with air creates problems when circulated to the quench bath. Addition of make-up methanol to the regeneration bath at 30 and cooling the recirculating solution at 45 serves to minimize the problems.

Should the problems still remain an auxiliary deaerator 49 near the end of conduit 40 should overcome the difficulties. The regeneration bath can be heated by heating the compressed air supply or by including a heating coil (not shown) in the regeneration bath. Although the quench bath regeneration system illustrated in the drawing shows the methyl formate and methanol being ducted to the atmosphere, it is well within the capacity of one skilled in the art to provide a system of recovering the same, for example, by using a chilling tower.

By determining the rate at which the contaminants are introduced into the quench bath, the allowable concentration of contaminants in the quench bath, the

size of the regeneration bath, the minimum residence.

100 cc formic acid 7 38.1 cc methyl formate 1l.l cc water i In the above composition. the formic acid is the polymer solvent whereas the methyl formate and water'are the non-solvents. Moreover, the dope'formulation used contained the following amounts of formic acid and methyl formate per cc'of nylon polymer dope solution: 100 cc formic acid/184 cc dope 054 cc formic acid/cc dope and y -38.l cc methyl formate/184 cc dope. 0.21 cc methyl formate/cc dope or the total expressed as formic acid 0.67 cc formic acid/cc dope The volume of dope cast per minute was calculated as follows:

Cross sectional area 36.8 cm. casting width X 0.0254 casting thickness 0.935 cm Casting rate 18.2 cm/min. Volume of dope cast per minute: 18.2 cm/min X 0.935 cm 17 cc/min. Volume of formic acid available to quench bath per minute is then:

0.67 cc formic acid/cc dope X 17 cc dope/min.

l 1.4 cc formic acid/min. The maximum allowable concentration of formic acid in the quench bath was arbitrarily set at a 1% level. the recycle flow rate, based on the 1% level of formic acid, was calculated as follows:

11.4 cc formic acid Min. 1140 cc/min.

Flow Rate divided by 0.01

84 liters mm 74 minutes A pilot plant size regeneration treatment system was constructed based on the foregoing calculations. The regeneration system consists of a tank 36 inches X 24 inches X inches deep, which is fitted with six porous polyethylene tubes installed on the bottom of the tank. The porous tubes covered with 1 foot of Duolite ES-26 cation exchange resin in the hydrogen form. In operation, the tank is filled with quench bath solution, and air is forced through the porous tubes creating a fluidized ion exchange resin bed, thereby providing extremely high contact surface between the resin and the solution in the regeneration tank. The formic acid is converted to methyl formate and removed from the tank via the air purge. The tank is covered with a closely fitted hood which carries the methyl formate vapors away from the tank.

The quench bath regeneration system was tested over a four-hour period under production conditions. The data obtained are shown in Table l. The data show that the regeneration ,tank was indeed controlling the methyl formate and formic acid level.

TABLE 1 5 QUENCH BATH/REGENERATION TANK r T ANALYSIS ON DRUM'RUN 7 Vol.-% Vol. 7: Elapsed Sample 1 Vol. I Vol. Methyl Formic Time Location' Water MeOH Formate Acid 0 QB. 50 50 .10 45 RB. 59 4'1 0.01 0.05

1 20 v0.13.. 51' c 47.8 .0.25 0.52 120 R.B. 59.5 40.5 0.04. 0.05 150 GB. 50.5 48l5 0.26 0.67 f 150 R.B." 59.5 40.5 0.04 0.05

180 0,13,, ,5 l .2 47.4 0.35 0 0.81 180 R.B. 59.6 40.2. 0.06 0.05 r 210 0.8. 52.3 46.2 0.48 1.02 210 :R.B.' 60.1 30.9 0.07 .0.05 240 0,3. 52.2 46.1 0.53 m2 240 KB. 60.7 39.2' 0.07 0.05

GB. Quench Bath Sample taken near casting bar RB. Regeneration Bath Sample taken at recycle of regeneration hath Although this invention has been described with respect to an improved polymer film process as the preferred mode of the invention, it is applicable to purifying solvent systems which are contaminated by formic acid, especially solvent systems containing methanol. In this aspect of the invention, the same considerations apply to the selection of appropriate conditions as with the non-solvent system for the film-forming polymer.

Many variations of the described process will be apparent to those skilled in the art without departing from the spirit and scope of this invention which is defined in the following claims.

What we claim is:

1. ln a method of preparing a film by casting a film of a dope solution comprised of a film-forming polymer in a solvent system comprising formic acid and quenching said film in a quench bath comprising a non-solvent system for said polymer, the improvement which comprises continuously contacting at least a portion of said non-solvent system with a heterogeneous esterification catalyst in the presence of methanol to form methyl formate by esterification of formic acid with said methanol and removing the methyl formate therefrom.

2. The method of claim 1 wherein the catalyst is a particulate strong acid ion exchange resin.

3. The method of claim 2 wherein the resin is fluidized by air.

4. The method of claim 1 wherein the methyl formate is removed by purging with an inert gas.

5. The method of claim 1 wherein the non-solvent system is heated at a temperature below the boiling temperature of methanol.

6. The method of claim 1 wherein the non-solvent system comprises methanol.

7. In a method of preparing a film by casting a film of a dope solution comprised of a film-forming polymer in a solvent system comprising formic acid, quenching said film in a quench bath comprising a non-solvent system for said polymer the improvement which comprises, removing from the quench bath predetermined amounts of the non-solvent system to a regeneration system, contacting the removed non-solvent system with a heterogeneous esterification catalyst in the presence of methanol to form methyl formate by esterification of formic acid with said methanol, removing methyl formate therefrom by volatilization and recycling predetermined amounts of the treated non-solvent system to the quench bath.

8. The method of claim 7 wherein the catalyst is a particulate strong acid ion exchange resin.

9. The method of claim 8 wherein the resin ized by air.

10. The method of claim 7 wherein the methyl formate is removed by purging with an inert gas.

11. The method of claim 7 wherein the removed non-solvent system is heated at a temperature below the boiling temperature of methanol.

12. The method of claim 7 wherein the non-solvent system comprises methanol.

13. A process for controlling a composition in a primary liquid system to which formic acid iscontinuously added, said process comprising continuously removing a pre-determined portion of the liquid to a zone in which it is contacted with a heterogeneous esterifica- 8 tion catalyst so that the formic acid therein is esterified in the presence of methanol to methyl formate and a substantial proportion of said methyl formate is removed; and continuously recycling the resultant formic acid deficient stream to the primary liquid system, such that the concentration of formic acid in said system is maintained at from less than about 1% up to about 5% by volume.

14. The method of claim 1, wherein said non-solvent is selected from the group consisting of water, methanol and ethanol, said film forming polymer is a nonalcohol soluble nylon, and said heterogeneous esterificatio'n catalyst is a strong acid or base ion exchange resin. 

1. IN A METHOD OF PREPARING A FILM BY CASTING A FILM OF A DOPE SOLUTION COMPRISES OF A FILM-FORMING POLYMER IN A SOLVENT SYSTEM COMPRISING FORMIC ACID AND QUENCHING SAID FILM IN A QUENCH BATH COMPRISING A NON-SOLVENT SYSTEM FOR SAID POLYMER, THE IMPROVEMENT WHICH COMPRISES CONTINUOUSLY CONTACTING AT LEAST A PORTION OF SAID NON-SOLVENT SYSTEM WITH A HETEROGENEOUS ESTERIFICATION CATALYST IN THE PRESENCE OF METHANOL TO FORM METYL FORMATE BY ESTERIFICATION OF FORMIC ACID WITH SAID METHANOL ND REMOVING THE METHYL FORMATE THEREFROM.
 2. The method of claim 1 wherein the catalyst is a particulate strong acid ion exchange resin.
 3. The method of claim 2 wherein the resin is fluidized by air.
 4. The method of claim 1 wherein the methyl formate is removed by purging with an inert gas.
 5. The method of claim 1 wherein the non-solvent system is heated at a temperature below the boiling temperature of methanol.
 6. The method of claim 1 wherein the non-solvent system comprises methanol.
 7. In a method of preparing a film by casting a film of a dope solution comprised of a film-forming polymer in a solvent system comprising formic acid, quenching said film in a quench bath comprising a non-solvent system for said polymer the improvement which comprises, removing from the quench bath predetermined amounts of the non-solvent system to a regeneration system, contacting the removed non-solvent system with a heterogeneous esterification catalyst in the presence of methanol to form methyl formate by esterification of formic acid with said methanol, removing methyl formate therefrom by volatilization and recycling predetermined amounts of the treated non-solvent system to the quench bath.
 8. The method of claim 7 wherein the catalyst is a particulate strong acid ion exchange resin.
 9. The method of claim 8 wherein the resin is fluidized by air.
 10. The method of claim 7 wherein the methyl formate is removed by purging with an inert gas.
 11. The method of claim 7 wherein the removed non-solvent system is heated at a temperature below the boiling temperature of methanol.
 12. The method of claim 7 wherein the non-solvent system comprises methanol.
 13. A process for controlling a composition in a primary liquid system to which formic acid is continuously added, said process comprising continuously removing a pre-determined portion of the liquid to a zone in which it is contacted with a heterogeneous esterification catalyst so that the formic acid therein is esterified in the presence of methanol to methyl formate and a substantial proportion of said methyl formate is removed; and continuously recycling the resultant formic acid deficient stream to the primary liquid system, such that the concentration of formic acid in said system is maintained at from less than about 1% up to about 5% by volume.
 14. The method of claim 1, wherein said non-solvent is selected from the group consisting of water, methanol and ethanol, said film forming polymer is a non-alcohol soluble nylon, and said heterogeneous esterification catalyst is a strong acid or base ion exchange resin. 